1. Field of the Invention
The present invention relates generally to optical fibers, and particularly to a method for distributed measurement of fundamental mode cutoff wavelengths in optical fibers. This method is especially suitable for measuring fundamental mode cutoff wavelengths of optical fibers which exhibit single polarization characteristics.
2. Technical Background
Optical fiber has become a favorite medium for telecommunications due to its high capacity and immunity to electrical noise. Single polarization optical fibers are useful for ultra-high speed transmission systems or for use as a coupler fiber for use with, and connection to, optical components (lasers, EDFAs, optical instruments, interferometric sensors, gyroscopes, etc.). Single polarization fibers (SPFs) propagate light in one, and only one, of two orthogonally polarized polarizations while suppressing the other polarization by increasing its transmission loss.
Single polarization fibers are specially designed optical fibers where each polarization mode reaches the cutoff wavelength at different wavelengths. The wavelength range from the wavelength that the first polarization mode reaches the cutoff to the wavelength that the second polarization mode reaches the cutoff is called single polarization band (SPB). Such single polarization fibers generally have an azimuthal asymmetry of the refractive index profile.
Different designs have been used to accomplish the single polarization performance. See for example, Michael J. Messerly et al, “A Broad-band single polarization optical fiber” J. Lightwave Technol. 9 (7), 817-820 (1991), J. R. Simpson et al, “A single-polarization fiber”, J. Lightwave Technol., LT-1 (2), 370-374 (1983); W. Eickhoff, “Stress-induced single-polarization single-mode fiber”, 7 (12) 629-631 (1982); Hirokazu Kubota, “Photonic Crystal Fiber”, Proc. Of SPIE, 292-297 (SPIE, Bellingham, Wash.); J. R. Folkenberg et al, “Broadband single-polarization photonic crystal fiber” (Optic Letters 30 (12); 1446-1448 (2005)).
The fiber cutoff wavelengths associated with each polarization mode is a critical fiber attribute that describe the performance of the single polarization fibers. Knowledge of fiber's fundamental mode cutoff wavelengths, the width of its single polarization band (SPB), and/or the central wavelength of the SPB are important factors in the implementation of the single polarization fiber in the state-of-art fiber optic transmission systems and devices.
It is well known that the physical properties of the optical fibers can vary as the fiber is being drawn. This influences optical properties, including the changes to the cutoff wavelengths and to the SPB. Thus, variability fundamental mode cutoff wavelengths within the length of the same fiber makes it difficult to predict the precise length of the fiber segment that needs to be cut off from the long length of fiber to provide the fiber with the predetermined cutoff wavelengths.
Measurement schemes for measuring fiber cutoff wavelengths are known. For example, in the above mentioned literature, the fundamental mode cutoff associated with each polarization mode are obtained through measuring the transmission spectrum of the fiber using broadband or white light source. The measurement is conducted in by cutting a short piece of fiber and performing the transmission measurement on this piece of fiber. This measurement technique is destructive because once cut of from the rest of the fiber, the measured section fiber is usually discarded.
Single Polarization fiber is manufactured to have its SPB in a specific wavelength range and thus has to have predetermined fundamental mode cutoff wavelengths. Unfortunately, screening fiber cutoff wavelengths on an entire length of fiber can not be done with the destructive measurement method. In addition, when the fiber cutoff wavelengths' values are beyond a specified range, the fiber is subsequently rejected. Conversely, when the fiber segment under test shows that the fundamental mode cutoff wavelengths are within the acceptable level, it is natural to assume that cutoff wavelength of the entire lengths of fiber are in the acceptable range. However, in reality, the fiber cutoff wavelengths change along the length of the fiber and consequently can vary from one segment along the length of the fiber to another. The whole fiber can then be considered as a concatenation of many segments of fibers. Thus, acceptable cutoff wavelengths measured on the fiber segment does not guarantee that the rest of the fiber has acceptable cutoff wavelengths. Therefore, there is a need for a different screening method that can take the distributed (i.e. variable) nature of fiber cutoff wavelengths into account.
Furthermore, the single polarization window is very sensitive to any change in fiber geometry such as core minor and major axes and (if the SP fiber utilizes air holes) hole diameter. It is very difficult to make a fiber preform and draw it into a standard fiber diameter such as 125 μm with a single polarization window right on target. It is necessary to change the draw conditions such as draw speed, tension and temperature, as well as pressure in the air hole to tune the single polarization window. One way of doing the tuning is to take a sample of the drawn fiber, measure its single polarization window, and then adjust the draw conditions accordingly. This process is repeated until the target single polarization window is reached. This process works well if the preform is perfectly uniform. However, a practical preform has always small random variations, which move the single polarization window significantly. As a result, the average yield of targeted single polarization fibers can be very low.
Accordingly, alternative methods that can conduct the measurements distributedly and non-destructively for identifying fiber cutoff wavelengths would be of great value to the industry in that such methods would reduce fiber costs, and therefore overall manufacturing costs for single polarization optical fibers. In addition, a process of making single polarization fiber with increased yield would be of great value to the industry.